Toggle linkage over-center float-operated valve actuating mechanisms are useful in controlling the filling of a closed or sealed pressure vessel and for driving an accumulated liquid periodically from the vessel. Typically, a vent valve vents the interior of the vessel to the atmosphere and a second pressure valve subjected to high pressure inlet fluids admits live steam to drive an accumulated liquid from the vessel. In filling the vessel, the vent valve is in the open position, and the pressure valve is closed by a single float-operated over-center toggle linkage valve actuating mechanism. As the vessel fills with liquid, the float rises and a pivotable float arm forming part of the valve actuating mechanism is swung or pivoted about a pivot point at an end remote from the float. That end is mechanically coupled to an over-center toggle linkage mechanism to move that mechanism towards a center, link in line position against a spring bias. Such mechanisms have toggle links which snap quickly through the center line position, attempting to close the vent valve simultaneously with snap opening of the high pressure inlet valve to pump the accumulated liquid from the vessel, and vice versa. Typically, a tension coil spring provides such biasing force, with the tension coil spring coupled at opposite ends to movable elements of the valve actuating mechanism; normally the opposite ends of two toggle links, which themselves are pivoted to each other remote from the spring coupling points.
Applicant's U.S. Pat. No. 5,141,405, issued Aug. 25, 1992, incorporates a compression coil spring under precompression as one of the toggle links to ensure that the open valve remains open and that the other closed valve remains closed on the valve seats until the precise moment that the toggle linkage moves over-center to change the state of both valves. In such mechanism, the pivot connection between one end of a compression coil spring (or dual compression coil springs), is pivotably coupled at a selected longitudinal position on a respective spring preload and swing arm (or arms) directly coupled to the float. In such case, the compression coil spring(s) is either additionally preloaded, i.e, compressed, or offloaded by expansion of the compression coil spring.
Such float operated over-center valve actuating mechanism requires the utilization of preload arms, is limited to the substitution of a compression coil spring for one of the solid metal toggle links and requires the selection of a spring constant for the compression coil spring correlated to the pressures encountered in liquid pumping systems which vary greatly, as do the liquids to be pumped, materially affecting the buoyancy forces acting on the float to cause over-center toggle linkage movement to change the state of the valves.
The applicant has ascertained certain problems when using a toggle linkage over-center valve actuating mechanism which employs one or several tension coil springs. Typically the spring tension is set by the factory to cover normal situations where the pump opens against a high pressure fluid such as steam. Typically manufacturers employ a heavy enough coil spring to open against 150 pounds of steam. To provide the desired utility to the pump, the manufacturer must provide a variety of springs, that is springs having a variety of spring constants. Additionally, in many of the toggle linkage over-center valve actuating mechanisms employing tension springs, the springs are slack when the toggle links are over-center, that is, they lack any force on the toggle linkage in the oppositely oblique, extreme over-center positions of the toggle links. Where the spring at opposite ends is connected to the ends of the toggle links remote from their center, common pivot connection, the spring stretches in both directions from its center point in the plane of the pivot connection between the links.
The applicant has further noted that since it is the float which causes valve actuation and over-center action, the mechanism does not sense the pressure within the interior of the pump housing; the valve opens only as a result or rise and drop of the accumulated liquid, which liquid must be pumped out by the driving fluid such as steam when that liquid reaches a predetermined level.
It is highly desirable therefore to have a toggle linkage over-center valve mechanism which works properly under conditions where the liquids to be pumped vary and where the gas pressure to affect that pumping may vary over a wide spectrum of pressures.
The applicant is also aware of the fact that upon drop in liquid level, the float is required to actuate the over-center toggle linkage. Such requires the dual links to pass through the center line position between the extreme pivot points at the opposite ends of the links, one of which is pivoted to a fixed frame, and the other to an element of the float and against the spring biasing force. Such reversing the state of the over-center valve actuating toggle linkage depends primarily on the mass of the float, and the float arm and its attendant elements coupled at an opposite end to a toggle link. Where the pump including the toggle linkage over-center valve actuating mechanism is designed to handle a liquid such as water, the pump may have difficulty in handling a naphtha because the buoyancy of the float by naphtha is quite different than that of water. In addition, where one may normally employ steam as the driving medium for opening the valve and to force the liquid to be pumped by the pressure fluid from the pump housing or other liquid container depending on the properties of the liquid, a different gas or pressure fluid may be required, such as nitrogen. Where one is pumping a liquid hydrocarbon or sulfuric acid, the selection of the gas would have to be one which is compatible with the liquid accumulating and requiring pumping, and in such case steam would not be applicable.
It is therefore a primary object of this invention to provide an improved, snap acting toggle linkage over-center valve actuating mechanism, of low cost, of simplified construction, which employs tension springs which may be preloaded, wherein the biasing force of the springs is maintained throughout the complete movement of the toggle linkage from one over-center position to the other, which permits onsite preloading of a tension coil spring or springs and which eliminates the necessity of correlation of the tension coil springs to the gas pressure of the drive fluid and the buoyancy force characteristics of the liquid to be pumped.